Some of the chemicals made sperm release too early a cocktail of digestive enzymes needed to break through the egg's outer coating. Photograph: Corbis

Common chemical additives found in sunscreens, food packaging, toothpaste and toys can harm sperm and may affect the fertility of some couples, scientists say.

Tests on 96 ubiquitous and supposedly non-toxic substances found that nearly one third disrupted the way sperm functioned, affecting their swimming and navigational skills, and their ability to fertilise an egg.

The findings – from an experiment conducted on sperm in dishes in the lab – are the first to demonstrate how the chemicals, which are so widespread they are detectable in people's blood, can impact on sperm and potentially harm reproduction.

Researchers in Denmark and Germany focused on chemicals that are known to mimic natural hormones in the body. Previous studies have linked such endocrine disrupting chemicals (EDCs) to fertility problems in the western world, but the paucity of human studies has left question marks over the harm they pose.

"This is really the first report on the direct effects of endocrine disruptors on sperm function," Niels Skakkebaek at Copenhagen University Hospital told the Guardian. "The effects we see could explain these not uncommon cases where people cannot have a child even if they are found to be normal by the usual examinations at fertility clinics."

With scientists at the University of Bonn, Skakkebaek tested EDCs used in a vast range of services and products. Some are added to sunscreens to filter out UV rays. Others make plastic food packaging easier to shape, or are given to livestock to make them grow faster. Still more are biocides, added to toothpastes, soaps, cosmetics and even children's toys.

In a series of experiments, the researchers added varying levels of the chemicals – singly and in combination – to dishes of around a quarter of a million sperm, to see what effect they had.

Around 30% of the chemicals led to abnormally high calcium levels in the sperm, which harmed their performance. Their swimming suffered and they released too early a cocktail of digestive enzymes that are needed to break through the egg's outer coating.

Skakkebaek said that EDCs in the female reproductive tract might also impair fertility by swamping the hormonal signal that sperm usually follow to find the egg. "Hormones produced by the egg tell the sperm where to find it, but if you have other chemicals mimicking that hormone, the 'GPS' on the sperm may tell them to go somewhere else," he said.

"This is a completely new aspect of research into endocrine disruptors. Like always, this must be repeated and expanded. We have examined only 96 chemicals and if there are 800 that may potentially be harmful, there's a lot of work to do," said Skakkebaek. The European Commission is reviewing its strategy on endocrine disruptors amid concerns over their health impacts.

One area that needs further investigation is highlighted in the scientists' report in the journal, EMBO. Tests showed that some chemicals had no noticeable effect at low doses, but when mixed with others at similarly low doses, the cocktail was harmful to sperm.

Allan Pacey, senior lecturer in andrology at Sheffield University, said the findings were "intriguing", but warned that it was too early to change advice to patients. "Although sperm calcium changes may be seen in the laboratory, this is a long way removed from what might happen in living people," he said.